drm: Nuke connector_list locking assert

I've forgotten to remove this when revamping the
connector_list locking.

Cc: seanpaul@chromium.org
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1483044517-5770-7-git-send-email-daniel.vetter@ffwll.ch